The field of the invention is the manufacture of hydrogen chloride and the present invention is particularly concerned with gaseous hydrogen chloride production from dilute aqueous hydrochloric acids.
The state of the art of the manufacture and processing of hydrochloric acid may be ascertained by reference to Kirk-Othmer, "Encyclopedia of Chemical Technology", 2nd ed., vol. 11 (1967) pp. 307-337, particularly pp. 314-315, where it is disclosed that 75% of U.S. hydrochloric acid production for the years 1961-66 came from chlorination processes, and pp. 315-323 under the section Gas Treatment and U.S. Pat. No. 4,115,530, the disclosures of which are incorporated herein.
In many commercial processes, hydrogen chloride is obtained as a by-product in the form of dilute aqueous hydrochloric acids which find industrial application in only a restricted way. In order to make use of this by-product hydrogen chloride which is present in its dilute aqueous form, for instance for the manufacture of vinyl chloride, ethyl chloride, chloroprene or chlorosulfonic acid, the hydrogen chloride must be isolated as a water-free gas.
The recovery of dilute acids required by distillation fails on economic grounds because of the excessive requirements for energy needed to evaporate the water. So far, the isolation of the HCl gas by extraction has failed because of the unfavorable distribution coefficients between HCl and the extracting means, for instance, the pentanols. Extractants with far more advantageous distribution coefficients are amines having substantially long chains. These amines and their hydrochlorides must be water insoluble. With these amines and hydrochlorides it is possible to separate almost completely the hydrogen chloride from dilute hydrochloric acid.
The ensuing thermal dissociation of the water-free aminohydrochlorides so obtained is carried out by the processes described in U.S. Pat. No. 4,115,530 and U.S. patent application Ser. No. 10,048.
The process of U.S. Pat. No. 4,155,530 provides a process for the production of gaseous hydrogen chloride by extracting dilute, aqueous hydrochloric acid with amines wherein:
(a) the aqueous hydrochloric acid is extracted by means of an amine or a mixture of an amine and an inert water-immiscible solvent having a boiling point lower than the amine used, the applicable amines being tertiary alkylamines, tertiary aryldialkylamines, secondary arylalkylamines, primary alkylarylamines or mixtures thereof, containing 14 to 36 carbon atoms in the nitrogen-bound side chains, or which at most one nitrogen-bound methyl group and at least one aliphatic group contains 6 carbon atoms, for which the acid constant K.sub.a of the amine is less than 10.sup.-3 ; PA1 (b) an inert, water-immiscible solvent having a boiling point lower than that of the amine is added to the extract unless the solvent already was added in stage (a); PA1 (c) the extract is distilled, the vapors generated are condensed, the water is continuously separated from the two-phase condensate and the organic phase is fed back into the distillation process; and PA1 (d) after removing the water, the extract is distilled under reflux at sump temperatures between 100.degree. C. and 250.degree. C. and the gaseous hydrogen chloride released at the head of the column is withdrawn. PA1 (a) heating solutions of amine hydrochlorides in an inert, organic essentially non-polar solvent at temperatures of about 120.degree.-230.degree. C., where the solvent has a boiling point at least 20.degree. C. above the temperature of heating;
According to the process of U.S. patent application Ser. No. 10,048, which is not yet part of the state of the art, the gaseous hydrogen chloride is prepared in a variation of the method of U.S. Pat. No. 4,115,530 by heating the mixture of aminohydrochloride and solvent to a temperature below the solvent's boiling point and by separating the hydrogen chloride being released by passing an inert flow of gas therethrough.
The process of U.S. patent application Ser. No. 10,048 includes the steps of:
(b) simultaneously with the heating, passing an inert gas stream through the heated solutions of (a) to split off hydrogen chloride and produce a mixture of hydrogen chloride and the inert gas; and
(c) separating the hydrogen chloride from the mixture of (b) wherein the amine components of the amine hydrochlorides are tertiary alkylamines, tertiary aryldialkylamines, secondary arylalkylamines, primary alkylarylamines or mixtures thereof, each of the amine components containing 14 to 36 carbon atoms in the side chains bonded to nitrogen, not more than one of the side chains being a methyl group bonded to nitrogen and at least one of the side chains being an aliphatic radical, bonded to nitrogen, containing at least 6 carbon atoms.
While the two processes of U.S. Pat. No. 4,115,530 and application Ser. No. 10,048 for the first time permit the preparation of hydrogen chloride from dilute aqueous hydrochloric acid with the use of special amines and subsequent thermolysis of the aminohydrochlorides formed, they nevertheless have some limitations. The main disadvantage is that the tertiary aliphatic amine dissociates in a shorter or longer period of time depending on the structure and on the magnitude of thermal loading. This dissociation forms such substances as primary and secondary amines and alkylchlorides and the dissociation efficiency drops thereby. While on the one hand primary and secondary aliphatic amines extract hydrogen chloride well out of dilute hydrochloric acid, on the other hand, they can hardly be dissociated thermally. Therefore, interfering by-products start forming initially in a hardly noticeable manner, but with time they progress ever more increasingly.
A process for preparing economical hydrogen chloride by extraction from dilute hydrochloric acid using amines can be economically implemented, however, only when the comparatively costly amine is recirculated with the least possible losses. This can be achieved for instance by using the method of U.S. patent application Ser. No. 64,633, which is not part of the prior art, by passing at least a portion of the mixture of amine, solvent and any contaminations over an adsorbent prior to use or before it is fed back into the extraction procedure.
Adsorptive amine purification, however, is costly on a commercial scale.